The invention relates to thermosetting moulding material based on an unsaturated polyester, a copolymerisable vinyl monomer, a catalyst, a shrinkage-reducing additive, and, if necessary, other usual additives.
It is known that moulding materials based on unsaturated polyesters in combination with copolymerisable vinyl monomers shrink to a greater or lesser degree when cured. As a result of this the surface of the resulting moulded articles prepared is not quite smooth, while the whole article tends to become warped and is no longer true to size. Further, this effect causes ribs, injection sprues and fibrous reinforcing agents to be marked on the surface. It has been proposed to oppose these disadvantages by adding a shrinkage-resisting additive (`low-profile additive`) to the system. It has already been proposed to add polyvinyl chloride, polyvinyl acetate, polylactones, polymethacrylates, polystyrene, copolymers of maleic acid anhydride and styrene, copolymers of acrylic acid or methacrylic acid with acrylates or methacrylates, and other thermoplasts, as `low-profile` additives. However, various additives bring the drawback that, though they oppose shrinkage, they are subject to exudation during the processing, so that the surface of the moulded articles is not smooth any more and, in some cases, even adheres to the mould. Also, a disadvantage occurring in case of coloured moulding materials is that owing to the phase separation between additive and polyester the coloration is no longer homogeneous, so that the moulded article shows stains and colour shifts.
The object of the invention is a shrinkage-resisting additive which does not show said disadvantages or to a much lesser degree.